For example, a disk-shaped optical recording medium (hereinafter, also referred to as an “optical disc”), such as a Blu-ray disc (registered trademark), has been known. Information is recorded on or reproduced from the optical recording medium, using laser light emitted from a semiconductor laser.
The recording of information on an optical disc by laser light is greatly affected by a variation in laser power due to temperature or a change over time, various skews or offsets caused by an adjustment error during manufacture, and a variation in recording conditions in drive control. Therefore, in particular, in a recordable optical disc, such as a write-once disc or a rewritable disc, a variation in a laser driving circuit or an optical element is prevented and accurate emission wavelength control is performed.
In general, before data recording is performed, a recording device searches for optimum laser power using a test area (optimum power control area (OPC area)) provided in each recording layer and adjusts recording laser power or strategy to optimize recording conditions.
In a test writing process (test write) for adjusting recording laser power, it is necessary to remove, for example, perturbation, to optimize the recording laser power, and to optimize a laser drive pulse in a state in which the optimum recording conditions are unclear. In some cases, laser light with energy more than is needed is emitted or laser light is emitted in a state in which the width of a laser drive pulse (laser emission time) is not appropriate in order to search for the optimum conditions such as recording laser power. As a result, the OPC area in the recording layer is likely to be severely damaged.
In addition, in a so-called multi-layer optical disc in which a plurality of recording layers are formed on a disc substrate, the recording or reproduction of data on or from a recording layer is affected by the recording state of another recording layer which is provided on the front side (which is closer to a laser light incident surface than the recording layer).
For example, the transmittance of the recording layer is changed by recording, which makes it difficult to irradiate the current recording layer with an appropriate amount of laser light. In addition, since a change in transmittance depends on recording power, it is difficult to control the change in transmittance, that is, the degree of influence on a rear-side recording layer in a portion in which recording is performed while recording power is changed, such as an OPC area.
From these points of view, there is a problem that it is difficult to achieve desired OPC according to the recording state of a front-side recording layer and to accurately derive optimization conditions. That is, in a case where laser power adjustment associated with test writing is performed in an OPC area of a recording layer, recording or reproduction is affected by OPC areas of other recording layers which are arranged at the same position (that is, the positions that overlap each other in a thickness direction (=a layer direction)) in a planar direction (a radial direction of the disc).
In contrast, in the related art, for example, a method is proposed in which OPC areas of different recording layers are shifted in the radial direction of the recording layer, as disclosed in Patent Document 1.
In the existing two-layer standard of the Blu-ray disc, the OPC areas of each recording layer arranged in a read-in zone which is provided on the inner circumferential side of the disc are arranged so as to be shifted in the radial direction of the recording layer.